Alkaptonuria is an infrequent autosomal recessive condition. It is first noticed in newborns when the urine in their diapers turns black upon exposure to air. The condition is caused by the defective transport of the amino acid phenylalanine through the intestinal walls during digestion. About 4 people per 1000 are carriers of alkaptonuria.
Sara and James had never heard of alkaptonuria and were shocked to discover that their first child had the condition. Sara's sister Mary and her husband, Frank, are planning to have a family and are concerned about the possibility of alkaptonuria in one of their children.
The four adults (Sara, James, Mary, and Frank) seek information from a neighbor who is a retired physician. After discussing their family histories, the neighbor says, 'I never took genetics, but I know from my many years in practice that Sara and James are both carriers of this recessive condition. Since their first child had the condition, there is a very low chance that the next child will also have it, because the odds of having two children with a recessive condition are very low. Mary and Frank have no chance of having a child with alkaptonuria because Frank has no family history of the condition.' The two couples each have babies and both babies have alkaptonuria.

What is the probability that the second child of Mary and Frank will have alkaptonuria?

Key Concepts

Autosomal Recessive Inheritance

Autosomal recessive inheritance occurs when a trait or disorder is expressed only when an individual has two copies of the recessive allele, one inherited from each parent. In the case of alkaptonuria, both parents must be carriers of the defective gene for their child to express the condition. If only one parent carries the allele, the child will not exhibit the disorder but may be a carrier.

Carrier Probability

A carrier is an individual who has one copy of a recessive allele and does not exhibit the trait or disorder. In this scenario, since Sara and James are both carriers of alkaptonuria, there is a 25% chance with each pregnancy that their child will inherit both recessive alleles and express the condition. Understanding carrier probability is crucial for assessing the risk of genetic conditions in offspring.

Family History and Genetic Risk

Family history plays a significant role in determining the genetic risk of conditions like alkaptonuria. If a family member has the condition, it increases the likelihood that other relatives may be carriers. In the case of Mary and Frank, Frank's lack of family history of alkaptonuria suggests a lower risk for their children, but it is essential to consider that he could still be a carrier, which would affect the probability of their children inheriting the condition.